Maturation at high frequency of germinal-vesicle-stage mouse oocytes after cryopreservation: alterations in cytoplasmic, nuclear, nucleolar and chromosomal structure and organization associated with vitrification.

After thawing and culture in vitro, greater than 90% of germinal-vesicle (GV)-stage mouse oocytes cryopreserved by vitrification, were capable of resuming meiosis and undergoing normal chromosomal and cytoplasmic maturation to metaphase II. This high frequency of development occurred against a background of profound alterations in the structure and organization of the cytoplasm, nucleus, nucleolus and chromatin during the dehydration stage of vitrification. Most, but not all, cytoplasmic and nuclear perturbations returned to a normal state during post-thaw culture. However, the results clearly demonstrate that vitrification is associated with chromosomal and cellular disorders that could adversely affect development after fertilization. Irreversible changes of potential developmental significance observed after vitrification at the GV stage include (i) premature chromosomal condensation, (ii) mixing of nucleoplasmic and cytoplasmic components prior to GV breakdown, and (iii) externalization of chromatin fragments into the cytoplasm after reformation of the oocyte nucleus, which shows the potential for the generation of fertilizable oocytes containing deleted segments of DNA.